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Phase 90 mods

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  • #16
    Originally posted by David Schwab
    I have a circuit board layout and parts list for a script Phase 90 I did some years ago. I had to repair one (an op amp went bad) so I took some time and drew it out. Later I also owned a script model that stopped working one day. Probably also been a part failure, but I never took the time to find the part.

    I think I know where I put the diagram... I'll have to look for it.

    I figured it might be a way to buy a new unit and mod it to script specs. I'd just build one myself, but I don't feel like checking/biasing all those FETs...

    I got it for ya

    From Nero1985, :


    Here you find out how to turn your Phase 90 that you just got a Guitar Center into the Script Phase 90 wich is not just a R28 mod or the C11 and C12 mods, it goes beyond that.
    There are 8 components on the new P90s that werent on the Script logo used by Eddie Van Halen and many others, here you can see what components have to be removed and wich ones replaced.
    Im also including a picture of its board with the component numbers so that u dont have to look for them all night long just to remove it or replace it!
    The Script Mod was tested by me and worked amazing, you'll get a very nice phase that might not be great as a pre distortion pedal but if u run it on ur Fx loop of after a Tube Screamer (like i do) you'll a crazy Phase!
    The OTHER MODS havent been tested but they should work fine.

    It took me a hours to realize the structure of the new P90 so i hope it helps you....

    You can't really mod a block logo or reissue P-90 to script specs, as the circuits are too different (# of chips, etc). And there were more than one version of the script logo, with different numbers of chips. MXR used revision numbers on the boards, i.e. rev 108 came out in 1978, which rev is your mod? My 1974 P90 (Bud Box) has 6 chips in it as does my later 76 script logo.
    in 1979 they changed from 741 chips (single op amps) to TL062 duals, so # of chips was reduced. You should replace the op amps with 1458 chips for a script mod.




    • #17
      I posted a reply.. and it vanished!

      I had the old version with six 741 op amps, and the Bud case.

      That's the one I diagrammed.

      On the one I repaired, since I couldn't find which 741 went bad, I pulled all of them, installed sockets, and replaced them with TL071's.

      Worked out real nice.

      OK... So maybe I'll just build my own.
      It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. Albert Einstein


      • #18
        Eastern Washington? Is that near the town of George? Just curious.

        Some folks want a phaser for wah-like cyclical emphasis, but some want it for gentler animation, and some - god help them - want it for both. That's why a **real** phaser has a control to vary it. The Small Stone had a 2-position Color switch that provided two resonance settings along with their corresponding sweep width setting. I suppose that gave it a big edge over the single-setting Phase 90. The MXR Phase 100 leapfrogged over that and provided a 4-position switch that yielded combinations of two sweep width settings and two resonance settings.


        • #19
          Mods to the Script 90

          I own a Script 90 that used to belong and used on stage by W. L. Van Zandt, of "Van Zandt" guitar pickups. He used to play lead for his cousin and a mentor of mine on the DFW area vfw circuit. He had wired in an old wall wart. I have since installed a socket for my boss power supply to run it.

          The hand traced circuit board has VZ pencilled in on one corner. I hate to mod it but I wont sell it, so if there is a slick way to to wire in a pot for level and perhaps depth control, I would sure like to know what the mods to the Script would be. Any ideas?

          Thanks in advance



          • #20
            Two kinds of "depth", or at least controls that might conceivably be labelled "depth". One is the contribution of the phase-shift channel to the mixed sound (sometimes called "Intensity" or Blend). The other is the width of the sweep. Even that can be split in different ways. It could be how high the LFO sweeps from some basic starting point. Alternatively, it could be how low the LFO sweeps from some starting point higher up.

            The Blend/Intensity sort of mod is easy to do. The P9 uses a pair of 150k resistors to passively mix the wet and dry signals to produce the cancellation-based notches. If you add to the basic 150k resistance on the wet side, the phase-shift effect will be less intense. A 250k pot, wired as a variable resistance, is likely sufficient to do the job.

            The other kind is a little trickier. Unfortunately, I know much less about it.

            Wiring up a volume control is a breeze, though. The standard P90 uses a 150k terminating resistor on the output. No reason on earth why that 150k resistor could not be replaced with a 100k log pot and a 51k fixed resistor from the ground lug of the pot to the circuit's ground. It would function essentially like a 150k pot that could only be turned down 2/3. Not really a problem, though. Naturally, instead of taking the output signal from the junction of the .05uf cap and 150k resistor, you would connect the pot to the cap and take the output from the wiper of the pot.


            • #21
              Thanks Mark, I will investigate that when I get time to look into it. This forum is a wealth of information.

              Keep On Pickin,